1. Field of the Invention
The present invention relates to a power amplification device, and is applied, for example, to a power amplification device using a BTL (Bridge Tied Load) system.
2. Description of the Related Art
Conventionally, there is known a BTL-system amplification device as a power amplification device that is used as an amplifier for a speaker of vehicle-mounted stereo equipment or other audio equipment (e.g. Jpn. Pat. Appln. KOKAI Publication No. 5-235652). As disclosed in KOKAI Publication No. 5-235652, a BTL-system amplifier circuit device includes a pair of amplification circuits (2) and (3), and thus both the voltage and current, which can be output to a speaker (4), can be doubled formally, and a high maximum output power can advantageously be obtained.
In the BTL-system amplifier circuit device, if an output terminal, which is connected to the speaker (4), is short-circuited to a ground or a power supply side (“short-to-ground” or “short-to-supply”), the amplifier causes an excessive current to flow in order to drive a short-circuit resistor thereof. Consequently, an output transistor (2b, etc.) of the amplifier circuit (2), (3) may be destroyed by an overload. To avoid this, there is a need to provide some protection means.
Whether the output transistor (2b, etc.) is destroyed or not is determined by whether a heat amount [heat W=power P=current I×voltage V], which is produced in the output transistor (2b, etc.), exceeds a predetermined tolerance value or not.
In the conventional protection means, however, a current flowing in the output transistor (2b, etc.) is let to flow through a current-mirrored detection transistor (20b, etc.), and the current is directly monitored, detected and compared. Thus, the conventional protection means is configured to detect only the current I among the factors that determine the heat amount. This being the case, there is a tendency that only ambiguous determination can be made as to whether the output transistor is destroyed or not.
As a result, the conventional device cannot sufficiently enhance the precision of sensitivity setting of the protection circuit, and is disadvantageous in improving the detection/protection sensitivity of short-to-ground or short-to-supply, thereby to prevent destruction of the amplifier circuit.
On the other hand, in order to realize the setting for improving the detection sensitivity of short-to-ground or short-to-supply, a drivable BTL load (RL) has to be increased, although this is a demerit. In short, the magnitude of the drivable load is restricted, and this is disadvantageous in maximizing the performance for driving the BLT load. As described above, there is a certain trade-off between the detection sensitivity of short-to-ground or short-to-supply and the load driving performance.